You can work out without using a gym (whether you can do it in your local park I'm not sure)
Do you have hills (hill reps - HIIT)
Do you have benches (pushups with your feet raised, dips etc)
Do you have trees or kids play bars (pulls ups, chin ups)
Do you have logs, rocks, other heavy things to haul around.

The main thing is that building mass requires resistance training, moving heavy things around. Weights that are round and come on bars are just easier and more convenient.

And you might get some odd looks doing hill reps with a log under each arm at 6am from the park wardens (I certainly did)